There is presently a demand in the aircraft industry for aluminum alloys which have high strength, high elastic modulus, low density and high corrosion resistance. For example, alloy 7075, a precipitation hardened alloy, is one of the current standards of the industry for various purposes. Aluminum alloys of higher strength and higher corrosion resistance than alloy 7075 are being sought, particularly for advanced designs. Because of the potential that the addition of lithium offers for improving properties of aluminum with respect to density and elastic modulus, several Al-Li containing alloy systems are presently under study. For example, T. H. Sanders and E. S. Balmuth have reported on three experimental alloys in "Metal Progress", pp. 32-37 (March 1978), viz. Al-Li containing 2.83 and 2.84 w/o (weight %) Li, Al-Cu-Li containing 1.5 w/o Li, and Al-Mg-Li containing 1.37 to 3.14 w/o Li. These alloys, which appear to be formed by "ingot metallurgy", i.e. from a melt, rely for their strength on the precipitation of the .delta.' phase, Al.sub.3 Li. The .delta.' phase coarsens at elevated temperature and transforms to the incoherent .delta. phase, less effective from the standpoint of strength of the alloy. It has been reported that the .delta.' phase is known to coarsen rapidly at temperatures of about 200.degree. C. This coarsening leads to lower stress rupture properties, and in general, would lead to lessened thermal stability. Furthermore, Al-Li alloys made by an ingot route suffer from severe oxidation during melting.
It has now been found that high strength, high specific modulus, dispersion strengthened Al-Li alloys which have good thermal stability can be made by a powder metallurgy technique known as mechanical alloying.
The mechanical alloying technique has been disclosed, for example, in U.S. Pat. Nos. 3,591,362; 3,740,210 and 3,816,080. These patents are incorporated herein by reference. Mechanical alloying, as described in the aforesaid patents, is a method for producing composite metal powders with a controlled, uniform fine microstructure. It occurs by the fracturing and rewelding of a mixture of powder particles during high energy impact milling, e.g., in an Attritor Grinding Mill. The process takes place entirely in the solid state. The repetitive cold welding and fracturing of the powder particles during mechanical alloying of the aluminum incorporates dispersoid materials, such as, for example, the naturally occurring oxides on the surface of the powder particles, into the interior of the composite powder particles. As the process continues the repetitive welding and fracturing of the powder particles, the incorporated dispersoid particles are reduced in size as they are homogeneously dispersed throughout the powder particles. In a similar fashion metallic alloy ingredients also are finely distributed within the powder particles. The powders produced by mechanical alloying are subsequently consolidated into bulk forms by various well known methods such as hot compaction followed by extrusion, rolling or forging.
U.S. Pat. Nos. 3,740,210 and 3,816,080 are specifically directed to mechanically alloyed aluminum systems and they disclose that one or more elements, among them Li, can be incorporated in the alloy system. By way of example, the patents mention that up to 1.5% lithium can be added. Various solubility limits of Li in Al at room temperature have been reported, e.g. 0.6, 0.7 and 1.5%. In the present invention, more than 1.5% Li is present, and there is lithium available to form a stable oxide. However, the level of Li is controlled to minimize or avoid phase precipitation which might lower the corrosion resistance of the alloy. Thus, the present invention enables the production of an aluminum alloy system with high strength, high specific modulus and excellent corrosion resistance and thermal stability. By thermal stability is meant that the room temperature strength is not affected by cycling to high temperature, i.e., a maximum of about 200.degree. C. and back to room temperature. Furthermore, it enables the production of Li-containing Al alloys by a method which avoids problems of handling lithium which are present in melt techniques for making the alloy.